If you have a drainage on your cage, I would just up the dosage and give a few longer mistings a day right in his cage to avoid the shower thing. I use different methods now. I have a few who loved it, and a few who really hated it... including me. Big waste of water, and people will tell you to leave your cham like this for 30 or so minutes while they are only benefiting from the mist bouncing off the walls.
How are you misting?
How often and for how long are you misting?
Do you have a dripper?
Why do you think your cham is dehydrated?
Dehydration is an easy thing to avoid and even treat without having to put your cham into a shower. Regular mistings throughout the day, a dripper, and some easy way to catch the water, and dehydration worries are a thing of your past. All the soap and stuff in the shower, and the loud strange environment for a cham... If you can get it done in their cage that is the place to do it. Every once in a while if I am worried about over watering the cages plants I will pick one and take the cham to the shower... But I use a Pump Pressure hand mister and set it there to provide the mist, come back in a few minutes to give t a few a more pumps then you are set, and using about 1/100th of the water you would use showering them with the shower head.
~Joe